1. Field of the Invention
This invention relates to a precision positioning device capable of effecting precision positioning of a workpiece and a working tool relative to each other with a high degree of accuracy in a machine tool, such as a grinder, lathe, etc.
2. Description of the Prior Art
In recent years, attempts have been made to achieve developments by leaps and bounds in industries concerned with peripheral equipment of computers, semiconductors, household electrical appliances, etc., for the purpose of producing articles of high performance, compact size, light weight and low cost. With this tendency, there has been created a demand for effecting positioning with a high degree of precision of parts of high hardness and high brittleness, such as magnetic heads, silicone wafers, etc.
For example, a magnetic head of a VTR is made of ferrite in the form of a rectangular parallelepiped having a width of 2.3 mm, a depth of 1.7 mm and a height of 0.14 mm, in approximate figures. However, owing to an increase in the density of recordings, the track width governing the density of recordings has increased in precision from 60.+-.3 .mu.m to 28.+-.2 .mu.m, and the present tendency is to obtain a further reduction in track width and to achieve a closer tolerance. Parts, such as magnetic heads, are very small in size as aforesaid and difficulties are encountered in handling them. Thus in production, it is usually effective practice to work on a large number of similar parts as by a grinder in a single operation and to separate them from one another after the parts are assembled. It is required, therefore, that in working on magnetic heads for obtaining correct track width, not only a single pitch but also the cumulative pitch have high precision, and in a machine tool, such as a grinder, it is essential that there be provided means for effecting precision positioning of a working tool and a workpiece.
As a mechanism for effecting precision positioning of a tool and a workpiece in a machine tool, it has been proposed to use means presently to be described for cutting a workpiece or forming grooves therein at arbitrarily selected spacing intervals.
When a workpiece is affixed to an X-Y table and such table is moved in a work indexing direction (an X-direction, for example) by means of a feed screw rotated by a motor, the amount of movement is measured accurately by means of a laser measuring system composed of a laser oscillator, an I/O interface, an interferometer, a reflector, a receiver, etc., and the measurements are fed back to a control of the motor, so as to thereby effect precision positioning.
However, some disadvantages are associated with this type of positioning mechanism. For example, in the aforesaid positioning mechanism, it is possible to measure the distance covered by the movement of the X-Y table in the work indexing direction with a high degree of precision by using a laser beam. However, it is difficult to determine accurately the distance between a workpiece and a working tool that governs the precision with which working is done or the precision with which indexing is performed, because changes in ambient temperature that might occur during working operation might cause variations to occur in the dimensions of a rotary shaft supporting the working tool or the X-Y table on which the workpiece is fixed.
Thus it is difficult to obtain precision positioning of a high degree by the aforesaid precision mechanism, with better than .+-.1.5 .mu.m and .+-.8 .mu.m being the limits for a single pitch and accumulated pitch respectively.
The result of this is that in a process step of working on a magnetic head for forming a track width, it is difficult to improve the precision with which working is carried out and the yield rate is no higher than about 98%. Thus inability to effect precision positioning is the main factor concerned in reducing productivity.